To do this quest, you have to kill the elementals surrounding each of the five pylons. All of the elementals have to die in order for the pylon to power down. When the last one dies, you heard a shattering sound and you'll see the pylon 'explode' and it won't glow bright bluish-white anymore. (It'll be an ugly white afterwards). The elementals surrounding the pylons are immune to arcane magic. They also hit rather hard so it's best if you don't take all of them down at a time. It's possible to pull 1-2 from the pylon without aggroing the others.

When you go in, you'll see a pylon right away. Be careful to not aggro the trees on both sides of the pylon, and watch out for the walking giant trees (But if you;'re going to kill the big tree boss, Tendris Warpwood, you should just kill the giant trees walking around. Tendris Warpwood will call them all to defend him if you engage in a fight with him.)

Then, walk upstairs to where the undead are (the place is relatively small, I'll assume you can see the upstairs are from the first pylon). There, they'll be two pylons, one on the left side, one on the right. Either way you go (it's like a rectangle shape, [~ == ~] the middle is a hole you cannot 'float' over and the pylons are marked here with a ~. Illyanna Ravenoak (boss) and her bear are walking around one side of the rectangle (the side away from the entrance) so you migth want to kill that side), you'll have to kill a mob of non-elite skeletons. Be careful! Once these skeletons are dead, they'll spawn five non-elite worms to attack whoever had the killing blow on the original skeleton. They have very little health, but should be taken down a.s.a.p. as five worms hitting one person for ~50hp per hit will kill anyone pretty quickly.
Also, don't split up! There are invisible monsters that appear once in a while and if your healer (or someone) gets ahead of the rest of the party, they'll probably die if no one's there to defend them.

Ok! After getting those two upstairs, go back down to where Tendris Warpwood is. He's a giant tree down the stairs of the first (entrace) level. You can't miss him. If you cant find him... (I don't know what to say..) At lvl 60, I was able to walk down the stairs, hugging the right side, and was able to get to the door to his right (You need the Crescant key to unlock it) without aggroing the boss.
So! open the door, and go through (watch out! There's two invisible.. uh.. thingys here). You should find yourself in a circular room with what looks like wind elementals and mana elementals. There's also Immol'thar and some person (hm.. I wonder who that person is..;D) inside a forcefield.
There's a pylons on both sides of the room. Power those down. :]ou should only have to kill a few wind elementals to get to them.

After that's done, Immol'thar is right there with no force field! You'll see more elementals walking around in a circle around the boss area. You might want to kill them to avoid dragging them into the boss fight, but it's not required.

Kill Immol'thar and then walk to a little tunnel in the same room. (It's to the right of how you got into the circular room.)
You'll find yourself in what looks like a library. Prince Tortheldrin is downstairs. Kill him and quest complete!

DON'T LEAVE THE PARTY!! Go back to turn in the quest and it'll tell you to get the chest next to Prince Tortheldrin for an additional item. Those invisible monsters you killed earlier in the hallway and next to the quest giver? They'll be there again.

So thereya go. Good luck :]

A little more clear:

First Pylon is in Capital Garden, directly after entrance.
Second and Third is on top floor od Court of the Highbornes: One North, one South.
Forth and Fifth is in Prison of Immol'thar.

This quest counts toward the Loremaster of Kalimdor achievement.

Just wanted to add that donot believe the new blizzard quest tracker. It tells us to turn in the quest at the dalarn crater in hillsbard. This is wrong, you need to turn in the quest back to the quest giver, inside dire maul.